Breeding ecology of radio-tagged White-winged Doves (Zenaida asiatica) in the Coastal Bend region of Texas.
White-winged Doves (Zenaida asiatica) in Texas have been undergoing an unprecedented northward range expansion over the past 30 years (George et al. 1991; Schwertner et al. 2000). Concurrent with range expansion has been a strong trend toward establishment of urban, year-round populations (Small & Waggerman 1999). Prior to 1980, the number of White-winged Doves in Texas north of the Lower Rio Grande Valley (LRGV) was considered negligible (Cottam & Trefethen 1968; George et al. 1994). Understanding the ecology, particularly nesting ecology, of these relatively new populations will aid in management of the overall population of this species (Small et al. 1989).
Hayslette & Hayslette (1999) documented breeding of White-winged Doves in the Coastal Bend of Texas outside their traditional breeding period of mid-May to mid-August. However, nesting birds were not marked and individual variation in nesting success and productivity could not be quantified. Recent studies have indicated that radio telemetry is a feasible means for studying doves in the U.S. (Schulz et al. 1998, 2001; Small et al. 2004).
METHODS AND MATERIALS
All doves used in this study were handled in accordance with guidelines established by U.S. Government's Principles for the Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training and methods were approved by Texas State University-San Marcos Animal Care and Use Committee, protocol number 5QEKCT.
White-winged Doves for radio-tagging were captured in Kingsville, Texas between 19 May and 9 June 2000 using standard wire funnel traps (92 by 60 by 15 cm) (Reeves et al. 1968). Kingsville has an estimated 27,000 breeding pairs of White-winged Doves (Waggerman & Lyon 2001). Ground reconnaissance was used to identify aggregations of White-winged Doves and establish trap sites. Based on these observations three trap sites were established on the Texas A & M University-Kingsville campus and four trap sites in neighborhoods < 1.5 km directly west of campus, encompassing an area 16 [km.sup.2]. The study area included all area within the incorporated city limits of Kingsville. White-winged Doves concentrated in residential areas and on the campus of Texas A & M University-Kingsville. These areas consistently exhibited similar characteristics of > 50% of Rio Grande ash (Fraxinus berlandieriana), oak (Quercus virginiana), hackberry (Celtis pallida), and mesquite (Prosopis glandulosa). Canopies were predominately open with an open understory consisting of turf grass lawns. Maximum canopy height was 15 m.
Seven trap sites were established and baited with a 3:1 mixture of commercial chicken scratch and black oil sunflower seeds (Purina Corp, St. Louis, MO). Four traps were used to capture White-winged Doves. Traps were rotated among trap sites to reduce trapping pressure at any one location. Traps were checked for captures every 90 min.
Transmitters used in this study were obtained from Advanced Telemetry Services (ATS, Insanti, MN). Transmitters weighed 3.5 g ([less than or equal to] 2.5% dove mass), measured 25 by 12 by 6 mm, and had a 15.2 cm antenna. Transmitters had a manufacturer's estimated operational life of 120 d. Mean unobstructed line of sight signal distance for transmitters placed 400 cm above the ground was 0.52 km (n = 40, SE = 0.01) as measured with an omni-directional antenna.
Forty adult White-winged doves were surgically implanted with subcutaneous radio transmitters and released at the trap sites (see Small et al. 2004). No reliable methods for aging White-winged Doves exist other than determining adults from hatching year birds. Estimated average handling time was about 15 min. Sex was determined using cloacal characters (Miller & Wagner 1955). Radio-tagged White-winged Doves were tracked from 19 May to 23 August 2000. An omni-directional antenna mounted on the roof of a vehicle was used to detect transmitter signals in a general area and a directional H-antenna located individuals to a specific site. Specific location points ([+ or -] 3 m) were determined using Global Positioning Systems (Garmin eTrexVista, Garmin Ltd., Olathe, KS) and classified as nest site or non-nest site. Locations were plotted on aerial photos, however, the number of unique points was too small to reliably estimate nesting home ranges.
Radio tagged White-winged Doves were randomly placed into five groups of eight birds each for tracking. Tracking was conducted daily from 0800 h to 1700 h with each member of a group tracked to a source location a minimum of twice every 14 days. Tracking times for individual doves in a group was rotated by 1 h each tracking period and tracking of groups was rotated weekly so each individual was tracked at varying times during ensuing days. This allowed us to collect location information across a range of diel and temporal periods for each individual.
Three hours of searching time was allocated for each group during a normal rotation. If an individual was not located within the initial rotation time, an extra hour of searching was conducted. If the individual was still not located, an extra hour of searching was conducted at night. If the dove was not located following an additional three days of searching, it was considered lost and removed from the search rotation, although its frequency was included while scanning for other individuals during night monitoring.
Doves located to nests were monitored every fourth day and nest status assessed using a mirror mounted on an extendable pole (Parker 1972). Species of nest tree, tree height, and nest height to the nearest 0.1 m were recorded. Differences in nest tree species and nest height were compared using goodness-of-fit tests with significance established as alpha [less than or equal to] 0.05 (Zar 1998).
Nesting success was calculated based on exposure days by the Mayfield method (Mayfield 1961; 1975) using a 14 d incubation period and 10 d nestling period established a priori. The Mayfield method was chosen because similar studies on White-winged Dove nesting success used this method or a variation of it (Hayslette et al. 1996; Hayslette & Hayslette 1999; Small et al. 2005) thus allowing for comparison. Hatch rate (which requires disturbing nesting individuals) was not measured due to concern of its potential influence on next success. Standard errors and 95% confidence intervals for nesting success were calculated (Johnson 1979). Because both parents participate in egg and nestling care with at least one adult present at all times (Cottam & Trefethen 1968), nests were considered active if at least one adult was present. In instances when no adult was present a mirror mounted on an extendable pole (Parker 1972) was used to verify that the nest was inactive (i.e., eggs or nestlings were no longer present). The likelihood of eggs surviving to hatching was considered equal to nestlings surviving to fledging because young are altricious and completely helpless (Small et al. 2005). Nests in which at least one fledgling survived to day 10 post-hatching were designated as successful. Nesting success was based on, and compared to, nest attempts. Nesting success was considered different if 95% confidence intervals did not overlap.
Mean surgery times (not including anesthesia inducement and recovery) was 8.04 min (n = 40, SE = 0.42 min, range = 4.88 - 15.45 min). Anesthesia inducement was standardized at 3 min for all individuals. Post surgery recovery was varied and not recorded because in most cases implant surgery on another individual was begun as soon as the previous surgery was completed. However, estimated handling time was about 15 min and did not exceed 25 min.
Individual White-winged Doves were monitored for a minimum of 27 and maximum of 96 d (n = 38, [bar.x] = 68.11 d, SE = 3.12). Doves were tracked to known locations 488 times (n = 38, [bar.x] = 12.84, SE = 0.85). Thirty-nine nests were located using radio telemetry. No implanted doves in this study were observed to have formed pair bonds with each other. Doves lost to domestic cat (Felis cattus) predation (n = 2) were excluded from analysis because time of mortality could not be definitively determined. Nineteen (73.1%) of 26 males and 7 (58.3%) of 12 females were observed nesting at least once. Of these, 9 (47.4%) of 19 males and 1 (14.3%) of 7 females that attempted a first nesting were observed making at least one more attempt. Of doves observed attempting 2 nestings, 3 (33.3%) of 9 males and no females (n = 1) made a third nesting attempt. There was a significantly greater proportion of males observed nesting (19) than females (7) ([chi square] = 5.54, P < 0.025). There was no difference in proportion of second attempts between males and females ([chi square] = 0.01, P = 0.40, Fisher's exact test) (Sokal & Rolf 1995).
Nests occurred in Rio Grande ash (12), oak (21), and hackberry (6). There was no significant difference among nesting tree species. Mean (SE) tree height, nest height, and nest height/tree height were 11.32 (0.31), 7.46 (0.32), and 0.66 (0.02), respectively.
Daily nesting success was 93% for first nest attempts, 16% for second nest attempts, 6% for third nest attempts, and 60% for all nest attempts combined. Nest success was significantly different between first and second nest attempts (Table 1). Six nests failed in the incubation stage and three nests failed in the brooding stage, all the result of abandonment for unknown reasons. No nests were reused, however, three doves re-nested in the same tree as the first nesting.
Previous studies of White-winged Dove nesting have been based on observations of unmarked individuals making comparisons between nesting attempts impossible because individuals could not be identified (Small et al. 1989; Hayslette & Tacha 1996). The use of radio telemetry in this study allowed for comparisons between nesting attempts by known individuals to be made.
Historically, White-winged Doves in the LRGV have relied heavily on anacua (Ehretia anacua), Texas ebony (Pithecellobium flexicaule), Mexican ash (Fraximus berlandieriana), and, to some extent mesquite (Prosopis glandulosa), as well as citrus for nesting (Cottam & Trefethen 1968, Schwertner et al. 2002). Tree species used for nesting differed from those traditionally used by White-wing Doves in the LRGV despite the presence of all of the traditional nest tree species in the study site (40 [km.sup.2]).
Although most reports of White-winged Dove nesting maintain that one or two clutches are attempted each breeding season (Cottam & Trefethen 1968, George et al. 1994), anecdotal evidence has suggested a greater number of attempts, particularly in urban, resident populations. Greater than two nesting attempts were, however, definitively determined to occur in this study and a subsequent study (Schaefer et al. 2004).
Daily nest success was similar to previous reports in urban, resident populations (Hayslette & Hayslette 1999; Small et al. 2005). A significantly greater proportion of males were found nesting than females, but the proportion that re-nested was not different between the sexes. This is likely because, in birds, there is generally a greater investment in time and energy expenditure for nesting by females (Whittow 2000). These differences may also suggest previously unknown gender differences, including a violation of the assumption of monogamy (Schwertner et al. 2002) in White-winged Doves. Categorization of doves as monogamous as defined by Lack (1968) and Wittenberger & Tilson (1980) is tenuous, and the term "apparent monogamy" is used for Columbids (Gowaty 1985). In addition, Blockstein & Westmoreland (1993) concluded that monogamy in the family Columbidae is based on few, mostly captive species with known exceptions. This study suggests that White-winged Dove natural history parameters in recently colonized populations differ greatly from White-winged Doves in their historic breeding range. Further research is warranted to determine how traditional and recently established populations differ in parameters not addressed in this study.
We thank M. G. Gray for his role in data collection. R. L. Small helped greatly in preparing the manuscript. An anonymous reviewer provided helpful comments. This study was funded by a grant from the Texas Parks and Wildlife Department White-winged Dove stamp fund.
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MFS at: email@example.com
Michael F. Small, John T. Baccus and Jay A. Roberson*
Department of Biology, Texas State University
San Marcos, Texas 78666 and
*Texas Parks and Wildlife Department
4200 Smith School Road, Austin, Texas 78744
Table 1. Mayfield method nest success indices (standard error) and 95% confidence intervals by nest attempt for White-winged Doves in Kingsville, Texas, 2000. Nest No. Exposure No. Nests Nest Success Attempts Days No. Nests Failed (SE) 95% CI 1st 324 26 1 0.928 (0.003) 0.925-0.932 2nd 81 10 6 0.158 (0.029) 0.129-0.187 3rd 18 3 2 0.059 (0.074) 0.000-0.133 All 423 39 9 0.597 (0.007) 0.590-0.604
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|Author:||Small, Michael F.; Baccus, John T.; Roberson, Jay A.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 2006|
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